1. Field of the Invention
The present invention relates to an optical system of the active range finder type.
2. Description of the Prior Art
An automatic focusing camera, including a light source and a light receiving element for scanning an object with light beams from the light source, and for receiving the light beams reflected from the object in order to focus, has been proposed in a variety of forms. For example, U.S. Pat. No. 3,443,502 discloses a method of projecting a modulated light beam for scanning, the scanning movement and a focus adjusting device being associated with each other, and stopping the focus adjusting device at a point where the light beam crosses a photo-electric light receiving element. Also disclosed is a method of flickering a plurality of light sources for carrying out scanning, and a method of selectively receiving modulated light beams from a fixed light source by a plurality of photo-electric light receiving elements.
A so-called field of view for range finding, in an active range finder of the above type, is determined by the size of the image of the light source projected onto an object by a beam projection lens.
Along with the recent technical development of light emitting diodes, and infrared light emitting diodes, such diodes have been used as a light source. While this type of element is of small size and has high efficiency, any image projected by it appears as a relatively small spot. Therefore, such an element can only provide spot range finding with a narrow field of view. It has the advantage of enabling precise focusing of an object being viewed, but when it is desired to focus on a particular part of the object, it is necessary to find a range, project the beam onto the part of the object and then to determine the composition of the picture frame before releasing the shutter, which constitutes added handling.
In order to widen the field of view for range finding, a beam projection lens may be made having a short focal length to provide a wide angle beam projection, but to obtain good range finding accuracy it is desirable to have a long focal length for the beam projection lens.
That is, referring to FIG. 1, when the amount of shifting of a light beam is expressed by "x"; a length of a base line between a beam projection lens and a beam receiving lens is represented by "a"; the focal length of the beam projection lens is expressed by "f.sub.1 "; while the length to an object is represented by "R", there is a relationship among these factors as shown below: EQU R=a.multidot.f.sub.1 /x, EQU dR/dx=-R.sub.2 /a.multidot.f.sub.1
When the length of base line a, and the accuracy of positional control for the amount of shifting of light source dx are constant, the accuracy of object distance dR is in an inverse proportion with f.sub.1.
That is, the longer the focal length, the higher the focusing accuracy becomes. Therefore, the field of view for range finding and the accuracy in range finding are two elements in an inverse relationship with each other.